These days, environmental regulations are being increasingly tightened on a global scale, among which fuel efficiency regulations and exhaust emission regulations for automobiles are especially being further tightened.
An important problem especially in a diesel engine has been how to reduce environment pollution caused by emission gas components such as particulate matters (PM) such as soot and NOx. An effective solution is to mount an exhaust purifying device such as a diesel particulate filter (DPF) or an exhaust purifying catalyst (oxidization or reduction catalyst) on an automobile.
On the other hand, lubricating oil used in an internal combustion engine is generally added with a metal-base detergent. Accordingly, when lubricating oil added with a metal-base detergent is used in an engine of an automobile on which a DPF as an exhaust purifying device is mounted, PM adhered to the DFR can be eliminated by oxidation or combustion. However, the DPF may be clogged by a metal oxide, a phosphoric salt and the like that are generated by combustion. Thus, reduction of such a metal-base detergent is demanded.
One effective solution for reducing fuel consumption of an automobile is to improve the automobile itself, for instance, by reducing size and weight of the automobile or by improving the engine. Another effective solution is to lower viscosity of lubricating oil so as to prevent friction loss of the engine. However, on the other hand, the lowering of the viscosity may cause portions of the engine to be more easily worn. In view of the above, a variety of additives are added to lubricating oil so as to prevent friction damages and wear of the engine entailed by lowered viscosity of the lubricating oil, among which ZnDTP (zinc dialkyldithiophosphate) is particularly known as an effective additive. ZnDTP, which is excellent in extreme-pressure properties and wear resistance, is widely used in lubricating oil for an internal combustion engine.
However, although exhibiting excellent performance on one hand, ZnDTP itself degrades to generate an acid material such as sulfuric acid or phosphoric acid, so that such an acid material reacts with a base component contained in the lubricating oil, thereby causing a decrease in base number thereof and shortening the life of the lubricating oil on the other hand. Further, while a three-way catalyst is used as the exhaust purifying catalyst for gasoline automobiles, the three-way catalyst is poisoned by phosphorous components contained in the lubricating oil. Thus, reduction of an additive containing phosphorus (e.g., ZnDTP) has been demanded.
With the above-described background, there has been a demand for lubricating oil containing neither metal-base detergent nor ZnDTP for use in an internal combustion engine, especially in a diesel engine.
However, drastic reduction of metal-base detergent has been difficult in view of long-drain capability, which is one of fundamental performances of the lubricating oil for internal combustion engines. Drastic reduction of ZnDTP has also been difficult in view of deterioration in wear resistance of engine valve portions. Lubricating oil using a specific disulfide compound as an antiwear agent has been also proposed (e.g., Patent Documents 1 to 3).    Patent Document 1: JP-A-2004-262964    Patent Document 2: JP-A-2004-262965    Patent Document 3: JP-A-2006-045336